1. Field of the Invention
The present invention generally relates to a digital demodulator suitable for wireless digital communication equipment and, in particular to a clock recovery control technique for the digital demodulator employing differential detection.
2. Description of the Related Art
As well known, .pi./4-shift quadrature phase-shift keying (QPSK) is, simply described, a form of QPSK modulation in which the QPSK signal constellation is shifted by 45 degrees each symbol interval. Therefore, the phase transitions from one symbol to the next are restricted to .+-..pi./4 and .+-.3.pi./4. Although the .pi./4-shift QPSK modulation can be implemented with coherent or differential detection, it is frequently implemented with differential detection because it is more simple, which is called differential .pi./4-shift QPSK, denoted simply as .pi./4-shift DQPSK. The .pi./4-shift DQPSK modulation scheme has been widely used for mobile communication equipment.
In such a conventional differential detector, however, when interference occurs, there are cases where a clock recovery circuit follows the interference wave, resulting in erroneous data decision.